In the oil and gas exploration and production industry many well operations require the use of tools which are deployed (and in some cases operated/controlled) into a well on wireline, such as electrically conducting wireline or non-conducting slickline or the like. Conventional wireline may comprise single or multi-strand steel cable, or alternatively may be formed of a composite material. Wireline operations may include well intervention procedures such as well logging to establish wellbore and formation conditions of a depleting well, or remedial operations, such as re-perforating and water shutoff, for instance. For example, a well fluid collecting/sampling tool may be deployed to formation depth on the end of a length of slickline which is translated by use of a winch system located at surface level.
A winch system for use with wireline typically includes a drum upon which the wireline is spooled in suitably pitched coils, as conventionally known in the art. A motor and braking system may be utilised to rotate and control the drum in either direction to deploy or retrieve a suitable tool coupled to the end of the wireline. It is possible, however, for the wireline to become jammed or entangled as it is paid out, due to the pitch of the coils and the fleeting motion of the wireline along the length of the drum. Jamming of the wireline in this manner may be minimised by, for example, maintaining a small maximum angle of fleet between the wireline and a wireline pulley system or initial lubricator tube or the like. However, due to the size of the winch, a small angle of fleet would typically only be achieved by locating the pulley system or lubricator or the like far removed from the drum, which may be impractical on offshore operations where space is at a premium, or where the winch system must be located with or on a subsea production system. Furthermore, it is essential that as the wireline is paid in, the pitch of the coils is maintained to prevent jamming of the winch system and to ensure that the wireline can be fully paid in to retrieve a tool from a well bore.
The past decade has seen the use of subsea production systems become the method of choice for exploiting offshore oil and gas fields. In the formative era of subsea production systems, it was envisaged that intervention operations would be conducted from a drilling rig or ship via a marine riser and Blow Out Preventer (BOP). However, the present Applicant has proposed the use of a self-contained well intervention system which can be deployed from a lightweight vessel and coupled directly to a wellhead, which offers significant advantages. Such a self-contained well intervention system is disclosed in Applicant's International Patent Application No. WO2004/065757. It is essential that the equipment utilised in such a self-contained intervention system be of a quality such that system integrity is not compromised, which would otherwise require continual retrieval and overhaul for maintenance and repair. It is therefore deemed essential that any wireline winching apparatus forming part of a self-contained intervention system is prevented from failure by seizing or jamming, particularly as the winching system may be exposed to wellbore pressures.
It is among the objects of the present invention to obviate or at least mitigate one or more of the disadvantages and problems noted above.